a) Field of the Invention
The invention is directed to a method for detecting defects during the inspection of structured surfaces, in particular masks, LCD's, printed circuit boards and semiconductor wafers, using specific image point features of the structures and defects in a recorded image of the surface in which zones of the image having similar image point features are compiled by image point classification and the behavior of the image point features of an image point is analyzed with reference to its neighboring image points by structure classification. The method is used predominantly in statistical process control in the production process.
b) Description of the Related Art
There are a great many methods for mechanical inspection of surfaces provided with structures. These methods can basically be distinguished as follows:
light scattering or laser scan methods, PA1 holographic methods, PA1 digital image processing methods.
In digital image processing, there are two basic approaches in solving inspection problems. A first variant is based on a comparison of reference values with actual values which, in turn, comprises three distinct methods:
1. Image-image comparison: the reference image is supplied by a test sample.
2. Image-data comparison: the reference image is supplied by an image synthesized from the CAD data record.
3. Data-data comparison. The comparison of reference values to actual values is effected on the basis of the CAD data.
Depending on the procedure, the effective principles of the first two methods require an extremely accurate positioning of the object support, two absolutely identical optoelectronic image channels and an extremely precise synthesis of sample images taking into account all physical and technical characteristics including tolerances ranging from illumination over the object, optical imaging, signal conversion in the sensor, analog channel to the virtual image.
In the third method, a complete analysis of the real image is required, i.e., in processing real-time images, trouble-free segmentation of the image, edge detection, approximation of structure elements and the description of the setting must likewise be achieved in real time.
All of the above-mentioned solutions based on comparison of actual and reference values are very cost-intensive due to the extremely high technical requirements for optics, precision mechanics, state detection and electronics (reference image generator).
On principle, these solutions can be used only for binary structures (masks, LCD's), lower structuring planes of process wafers and for comparison between simple repetitive structures.
In higher technological layers of the wafer, the characteristics of the layers and structure edges change due to technological processing and by overlapping and overlaying in such a way that the uncertainties in this area can no longer be overcome by comparing reference and actual values.
This manifests itself particularly with respect to changes in thickness in the individual layers (they lead to changes in grayscale value and color) and in overlay tolerances. All of these technological peculiarities add to the difficulty of comparison of reference values and actual values or render such comparison impossible. In addition, polysilicon and metallization layers are imaged as textured regions by means of statistical model parameters so that they are likewise inaccessible to comparison methods. Examples of such solutions are described in the patents mentioned in the following.
In an image-to-image comparison according to DE 2508992, an actual structure is compared with a second actual structure by means of split optical beam paths. The two images can also be superimposed on one another in the form of a video image and the combined image is then evaluated. In this method, structural defects are determined from mismatches in the covering of the two combined images.
In another method according to DE 2700252, the structure to be tested is raster-scanned by a test beam and broken up into image points. The total image which is digitized in this way is fed to a data processing system in which the scanned structure is compared with a reference image, reference structure or structure rules contained in a storage.
EP 0426 182 discloses a combined method which describes the generally known image-image comparison of two adjacent control objects on the one hand and the image-image comparison on a control object on the other hand. This method makes use of the fact that in certain classes of highly integrated circuits (storages, CCD arrays and other multielement sensors) structure elements are repeated on a chip many times.
Further examples for such a comparison of reference values to actual values are described in the following patents: DE 3336470, DE 3612268, DE 3714011, DE 4136830.
The second variant makes use of technological peculiarities by proceeding from a detection of defects on the basis of their specific features resulting from interference and deviations in the parameters of the technological processes, material defects, low-quality process materials, impurities and imperfections in the crystal structure.
In contrast to comparison testing, this direct detection of defects makes do, to a great extent, without extensive information on the reference structure, since it proceeds from determined fundamental rules or deviations from rules in detecting the image. Such rules and aberrations are provided substantially by color features and shape features of the reference structure, reference layer or defect.
One of these methods, described in DE Patent 3427981, proceeds from the fact that a minimum structure spacing can be defined on every structured control object and that a large number of the defects present on the control object have dimensions greater than or smaller than the given dimensions of the structure. Therefore, in an automatic inspection according to this patent, a measurement window can be used which measures a geometric structure as falling short of or exceeding the given structure dimensions.
Accordingly, these measurement windows detect only those structure elements lying within the given measurement window and not those which match the defined structure dimensions.
A disadvantage consists in that, in higher structuring planes of a wafer, a minimum spacing can no longer be defined by the technological peculiarities, in particular by overlaying different structure components, and the method is therefore only practicable for binary structures.